Superheater Tubes In Boilers Research Articles

Analysis of Ovality Deviation of Bending Superheater Tube Boiler Fabrication Process with Six Sigma Method in Manufacturing Company

Quality control in fabrication is critical to the success of the production process. In this study, key factors in production quality control were identified by analyzing the influence on the tube bending process in fabrication work. Ovality deviations occur during the tube bending process that exceeds 9 percent. 30 boiler superheater tube elements had to go through the repair process. The causes of tube bending deviations were analyzed using the Six Sigma approach with the DMAIC (Define, Measure, Analyze, Improve, and Control) methodology. The causes of failure were analyzed using FMEA, and the highest RPN of 320 was obtained due to design planning and manufacturing of bending dies that are in accordance with the OD tube and the placement of clamp die and pressure die distances that are not too far apart. In addition, an RPN value of 294 was obtained due to the bending speed being too high. The recommendation for the next project is to consider using a C-Frame machine with better control to make it easier for operators to adjust the pressure, temperature, and speed of a good machine to better maintain ovality.

Breakdown of Protective Cr-Rich Oxide Scale Formed on Heat-Resistant Steels for Superheater Tubes in a Waste Power Generation Boiler

Breakdown of Protective Cr-Rich Oxide Scale Formed on Heat-Resistant Steels for Superheater Tubes in a Waste Power Generation Boiler

Development of an HTS-SQUID Based Nondestructive Evaluation System for Boiler Tubes On-Site Inspection in Thermal Power Plant

In ultra-supercritical (USC) thermal power plants, Super304H (KA-SUS304J1HTB, ASME code: 2328) is widely used in the superheater and reheater tubes in boilers because of its excellent creep rupture strength under high-temperature and -pressure conditions. Thus, a degradation diagnosis of these tubes after long-term operation should be conducted using a quick and easy inspection. In this study, in order to discover the degradation of the boiler tubes for early diagnose before they rupture, a high-temperature superconductor (HTS)–superconducting quantum interference device (SQUID) with high sensitivity was used. A SQUID-based nondestructive evaluation system for the on-site inspection of boiler tubes in an USC thermal power plant was developed. Our system comprises a movable eddy current testing (ECT) probe and HTS-SQUID gradiometer. Five LED markers were set on a manually operated ECT probe for position detection using a stereo camera. Customized software was used for data acquisition through a lock-in amplifier and data visualization on a PC with a connected smart glass. Thus, the system can be used for quick inspection by time-sequence signals and detailed scanning through a two-dimensional contour map projected on the smart glass. Therefore, this study successfully introduced an HTS-SQUID-based nondestructive evaluation system for the on-site inspection of boiler tubes.

Failure Analysis of Utility Boiler Superheater Tubes of A213 T12

Failure Analysis of Utility Boiler Superheater Tubes of A213 T12

Diagnostics of power boiler superheater tubes made of austenitic steel using the metal magnetic memory technique

Diagnostics of power boiler superheater tubes made of austenitic steel using the metal magnetic memory technique

Over-attemperation and Short-term Overheating in Pendant-type Superheaters

ABSTRACT A water-wedge is often suspected to be the root cause for short-term overheating in fossil-fuelled boiler superheaters. However, it can be argued that evaporation of the water-wedge would cool the tube sufficiently and prevent overheating. This study aims to determine if the thermo-physical conditions occurring at low loads support this claim by studying the transient behaviour of a representative superheater segment under postulated conditions. A flow model was constructed to facilitate direct comparison with a boiler pendant superheater of a full-scale fossil-fuelled power plant. Several scenarios of water-wedges sustained by attemperation spraywater were simulated at low load operating conditions. The temperature evolution of the tube wall was tracked and, together with calculated equivalent stresses including thermal stress, was compared to the yield strength of the material. The results show that the stresses exerted over the tube wall and throughout the tube length are not sufficient to overcome the yield strength of the tube material, even for an aged tube under severe process conditions of boiler overfiring. Evaporation of the water-wedge provides sufficient cooling to the superheater tube to prevent failure. It was concluded that water-wedging alone is unlikely to be the root cause of short-term overheating at low boiler loads. Additional keywords: Short-term overheating; water-wedge; boiler superheater tube; attemperation; thermal stress, evaporation.

Failure Analysis and Remedial Solution Suggestion for Superheater Tubes of a Power Plant Boiler

Superheater tubes are the most critical components of a power plant’s boiler. Due to operation in high-temperature and -pressure conditions, platen superheater (primary superheater) tubes in thermal power point boilers are exposed to breakdowns such as creep failure and overheating. Therefore, identifying the causes of these failures and its maintenance is very important. Platen superheater tubes have failed in several similar power plant units at the same specific area. This problem is analyzed by three methods to find out the reason. These methods are metallography, measuring oxide layer thickness and CFD. These methods prove that the temperature difference between tubes is the cause of failure at the critical area. Length difference between tubes is the reason of the temperature raising in this case. Then, three solution methods are presented, namely (1) changing tubes material, (2) balancing tubes length and redesigning the platen superheater and (3) replacing the failed tube with new one. For comparing the methods, balancing tube’s length and redesigning is selected. The selected solution introduces three proposed plans. These plans will be alternative for the failed platen superheater. A comparison made between three proposed design and platen superheater indicates that all of the three proposed designs are better in temperature side than the existing platen superheater. After that, the best design is chosen by the CFD results. The selected design here is the proposed plan 2 because it reduces the 25 K temperature difference of the platen superheater to 5 K and this will prevent this kind of failing.

Investigation of Premature Failure of a Coal-Fired Boiler Superheater Tube

A metallurgical investigation has been carried out to ascertain the cause of premature failure of boiler superheater tube. The tube material was SA213-T22-grade steel. Visual observation of fractured region of failed component revealed thick-lip fracture, parallel cracks along the longitudinal direction, bulging, and non-uniform wall thickness. Microstructural investigation showed complete spheroidization of second phase, formation of thick scale to the tune of ~ 1 mm, the presence of creep voids, and significant decrease in hardness of the tube material. These signatures are indicative of overheating and creep of the alloy during service exploitation. Fracture surface predominantly contained the signature of intergranular failure. The microstructure near-fracture region exhibited a large number of creep voids and their coalescence along grain boundaries. Under the hoop’s stress, the coalescence of these creep voids triggered intergranular fracture leading to rupture of the tube. Operating temperature was estimated from steam side oxide-scale thickness using kinetic model equations. It indicated that for the formation of ~ 1-mm scale over a service span of 9 months required operating temperature of ~ 677 °C. However, the material is designed to withstand a maximum temperature of 540 °C. It was thereby concluded that overheating during the operation led to accelerated creep and subsequent failure within a short period.

Biomass fly ash deposition in an entrained flow reactor

Fly ash deposition on boiler surfaces is a major operational problem encountered in biomass-fired boilers. Understanding deposit formation, and developing modelling tools, will allow improvements in boiler efficiency and availability. In this study, deposit formation of a model biomass ash species (K2Si4O9) on steel tubes, was investigated in a lab-scale Entrained Flow Reactor. K2Si4O9 was injected into the reactor, to form deposits on an air-cooled probe, simulating deposit formation on superheater tubes in boilers. The influence of flue gas temperature (589 – 968°C), probe surface temperature (300 – 550°C), flue gas velocity (0.7 – 3.5 m/s), fly ash flux (10,000 – 40,000 g/m2h), and probe residence time (up to 60 min) was investigated. The results revealed that increasing flue gas temperature and probe surface temperature increased the sticking probability of the fly ash particles, thereby increasing the rate of deposit formation. However, increasing flue gas velocity resulted in a decrease in the deposit formation rate, due to increased particle rebound. Furthermore, the deposit formation rate increased with probe residence time and fly ash flux. Inertial impaction was the primary mechanism of deposit formation, forming deposits only on the upstream side of the steel tube. A mechanistic model was developed for predicting deposit formation in the reactor. Deposit formation by thermophoresis and inertial impaction was incorporated into the model, and the sticking probability of the ash particles was estimated by accounting for energy dissipation due to particle deformation. The model reasonably predicted the influence of flue gas temperature and fly ash flux on the deposit formation rate.

Failure Analysis of Superheater Tubes Cracking of a Boiler in a Power Plant

Abstract A failure analysis was performed on a power plant’s boiler superheater tubes that had burst. The macromorphology, chemical composition, microstructure, and mechanical properties were analyzed by experimental testing. Results showed that the superheater tube bursts can be classified as creep ductile cracks because the two tubes operated in overtemperature and low heat transfer efficiency conditions for a long time, and one of the tubes did not meet the material requirements. Finally, preventative measures were put forward.

Vpliv škaje na parni strani cevi izmenjevalnika toplote na dobo trajanja cevi zaradi lezenja

Vpliv škaje na parni strani cevi izmenjevalnika toplote na dobo trajanja cevi zaradi lezenja

Prediction of wall temperature and oxide scale thickness of ferritic–martensitic steel superheater tubes

Based on the thermal deviation theory and local energy and mass balance of the superheater tube, a new method of calculating tube wall temperature is proposed. The wall temperature distribution and the thickness of oxide scale of the superheater tubes of a supercritical boiler are estimated over the length of the steam flow path for different periods of service. This calculation of oxide scales on the steam side takes into account ash deposits on the flue gas side. The results of oxide scale thickness of boiler superheater tubes for different exposure time are compared with field measurement data. There is a sudden increase in the wall temperature at the tube elbow (position N) because of high gas temperature and high heat transfer coefficient. The distribution of the gas temperature field along the height of the heating surface has a great influence on the final wall temperature calculation results.

Behaviour of T91 Steel Oxide Scale Exposure to Temperature at 540°C for Different Exposure Duration

Grade T91 steel is one of a common material used for construction of superheater tubes in power generation boiler. In general, superheater tubes that operates in subcritical boiler are expose to operating temperatures in the range between 540°C to 580°C. Exposure to high temperature makes this steel subject to formation of oxide scale which can cause detrimental effect on the material properties. Excessive formation of oxide scale on internal surface of superheater tube can cause overheating damage at early stage of the boiler operation. This study was conducted to examine the behaviour of oxide scale forms on T91 steel for specimens exposed to temperature of 540°C for 200, 400, 600, 800 and 1000 hours holding time. The oxidation kinetic was identified as parabolic and the oxide scale was observed having 3 different layers which are haematite (Fe2O3), magnetite (Fe3O4) and spinel (Fe,Cr)2O3. The oxide scales layers were observed separated by voids and crack which are growing in size over time.

High-Temperature Corrosion Properties of Boiler Steels under a Simulated High-Chlorine Coal-Firing Atmosphere

High-temperature corrosion of the superheater and reheater tubes in utility boilers frequently occurs upon firing high-chlorine coal, which severely impacts the safety of the boiler operation. In this work, the corrosion mechanism of several commonly used boiler steels, including T91, 12Cr1MoVG, and TP347H, was investigated. First, the types and amounts of the main corrosive elements in the flue gas and the heating surface deposits from high- and low-chlorine coals were predicted with the aid of thermodynamic equilibrium modeling. The main corrosive elements that vaporized and subsequently condensed from the flue gas were determined to be Na, Cl, and S. Varying the wall temperature of the heating surface did not dramatically change the forms of these elements but slightly influenced their amounts. Second, lab-scale experiments were conducted in a tube furnace to estimate the degree of corrosion of various boiler steels under simulated conditions upon firing high- and low-chlorine coals. A sophisticated wa...

Experimental studies on fly-ash erosion behavior of Ni-Cr based nanostructured thermal spray coating in boiler tubes

ABSTRACTErosion study was performed on the uncoated and coated SA210Gr-C boiler tube substrates. This study was performed in a view to reducing the scaling on boiler water-wall and super-heater tubes. In the present work, the High Velocity Oxygen Fuel (HVOF) coating process was used to deposit nanostructured and conventional coatings on the boiler tube substrates. Fly-ash produced after the combustion of coal in an Indian thermal power station was used as erodent. The erosion loss of coated and uncoated substrates was recorded at a regular interval of 30 minutes. The study was conducted at 30°, 60°, and 90° impingement angles and at two different operating temperatures (300°C, 550°C). Surface morphology of eroded samples was analyzed using macro and micro imaging techniques, and surface roughness was also recorded for the eroded samples. The phases present in the eroded substrates were indexed using the X-Ray diffraction (XRD) analysis. HRTEM images of nanostructured coated samples at 550°C with varying impingement angles were analyzed. The result reveals that the nanostructure in the coating was retained even after high-temperature erosion test. Comparative study was made with nanostructured and conventional coatings against the uncoated materials.

天然ゼオライト粉末を用いた高効率廃棄物発電ボイラ過熱器管の高温腐食抑制技術

天然ゼオライト粉末を用いた高効率廃棄物発電ボイラ過熱器管の高温腐食抑制技術

Corrosion and life assessment of IntrexTM superheater tubes in a CFB oil shale boiler

Heating surfaces in oil shale boilers are subject to intensive high temperature corrosion (HTC). The protective oxide scale formed on austenitic steels SA213-TP347HFG, X8CrNiMoNb16-16 and X8CrNiMoVNb16-13 has been investigated and analysed (1) under both controlled laboratory conditions and (2) in the IntrexTM superheater of the circulating fluidized bed (CFB) boiler operating at the Narva Thermal Power Plant. Results show that the associated protective oxide scale tends to spall from tube surfaces. A long term investigation of HTC on the oil shale boiler tubes has demonstrated the applicability of these materials for such use and a comparison of HTC tubes manufactured from differing steel grades allows a selection of the most suitable grade. Remaining life assessments for particular tubes have also been conducted on the basis of this long term study of HTC on CFB boiler heating surfaces. This prognosis is vitally important to warranty safe and reliable further operation of thermal equipment.

Failure analysis and retrofitting of superheater tubes in utility boiler

The extreme spray water mass flow rate deviation was observed to occur in the middle temperature superheater of Sahand 2×325MW Power Plant utility boiler, which severely affected its economic performance and safe operation. Boilers operating in these conditions led to failure in superheater tubes at the same place for two consecutive times in a three year span. Thus, the failure analysis of superheater tubes by investigating the visual inspection, chemical, scale and creep analysis was carried out. The brittle failure occurred in the superheater tubes after the fuel was changed from natural gas to heavy oil. Failure analysis showed that tubes were suffering from long term overheating which was instigated by high spray water flow rate. In order to rectify the boiler operating conditions, some modifications were applied in the boiler unit 1 and operating parameters on this boiler were compared with boiler unit 2. The results showed that the 8.33% reduction in heating surface area corresponds to 52.84 and 17.80% reduction in spray water mass flow rate for capacities equal to 300 and 260MW, respectively.

Acoustic emission analysis on tensile failure of steam-side oxide scales formed on T22 alloy superheater tubes

Failure of steam-side oxide scales on boiler tubes can seriously influence the safety of coal-fired power plants. Uniaxial tensile tests employing acoustic emission (AE) monitoring were performed, in this work, to investigate the failure behavior of steam-side oxide scales on T22 alloy boiler superheater tubes. The characteristic frequency spectra of the captured AE signals were obtained by performing fast Fourier transform. Three distinct peak frequency bands, 100-170, 175-250, and 280-390 kHz, encountered in different testing stages were identified in the frequency spectra, which were confirmed to, respectively, correspond to substrate plastic deformation, oxide vertical cracking, and oxide spalling with the aid of scanning electronic microscopy observations, and can thus be used for distinguishing different oxide failure mechanisms. Finally, the critical cracking strain of the oxide scale and the interfacial shear strength of the oxide/substrate interface were estimated, which are the critical parameters urgently desired for modeling the failure behavior of steam-side oxide scales on boiler tubes of coal-fired power plants.

Failure evaluation of steam‐side oxide scales in superheater tubes during unsteady thermal processes: A probabilistic method

Issues resulting from failure of steam‐side oxide scales in boiler superheater or reheater tubes seriously influence the safety of coal‐fired power plants. In this work, an analysis model for the failure of steam‐side oxide scales during unsteady thermal processes is established. This model employs the advanced oxide scale failure diagram as the failure criterion, and also takes into account the influence of temperature on the coefficients of thermal expansion of both metal substrate and oxide scale. Since the stochastic nature of related geometrical, mechanical, and thermophysical parameters may hinder the usual deterministic investigation from obtaining a reliable evaluation, probabilistic failure evaluation which is not based on bivalent logic is thus performed. Focusing on the unsteady thermal processes induced by the variation of steam temperature, effects of oxide scale thickness as well as different steam temperature variation processes on the failure probability of steam‐side oxide scales are analyzed. It is found that the failure probability of thicker steam‐side oxide scales is higher, while through reasonably selecting steam temperature variation mode and appropriately lengthening variation duration, lower failure probability can be achieved. Feasible measures are presented with the aspiration to solve the steam‐side oxide scale failure issues.